A separator for storage battery such as a separator for valve regulated lead-acid battery is required to act as an insulating material that is interposed between a positive electrode and a negative electrode to separate the two electrodes from each other and act to retain an electrolyte. To this end, a storage battery is produced from a separator for storage battery mainly composed of ultrafine glass fibers which are excellent in acid resistance, oxidation resistance and hydrophilicity and have an average fiber diameter of from about 0.6 to 2 μm capable of increasing porosity. In a storage battery comprising a separator mainly composed of such microfibrous glass having a high electrolyte retention, when an electrolyte is injected into the battery case, the frictional force of the glass fibers with each other is reduced to cause the glass fibers to move, weakening the sheet structure and hence causing the pressure of the separator against the electrode to drop from that of the initial stage of assembly of storage battery. When the pressure runs short, the adhesion between the separator and the electrode deteriorates, making the reduction of battery capacity and life unavoidable. It is thus necessary in assembly of storage battery that an electrode group comprising a separator interposed between electrodes be previously pressed before being incorporated in the battery case so that the pressure can be kept as high as possible even after the injection of the electrolyte, and this was disadvantageous in that the pressure required for incorporation of electrode group in battery is as high as from 49 to 98 kPa, troubling the battery assembly and hence deteriorating productivity.
In order to cope with these problems, for example, JP-A-59-138059 and JP-A-7-122291 propose a storage battery which is arranged such that the electrode is pressed by the expansion of a separator caused by heating, taking into account the fact that when subjected to repeated cycle of charge and discharge, the positive electrode of a storage battery expands and shrinks to undergo volumetric change that makes the bond of particles of active material constituting the positive electrode each other relax, atomizes and softens the particles and causes the particles to be exfoliative.
Referring to the storage batteries proposed in the above cited patent references, a valve regulated lead-acid battery is produced by putting an electrode group comprising a separator interposed between a positive electrode and a negative electrode in a battery case, heating the assembly to allow the hollow body in the separator to expand so that a pressure is applied to the positive electrode, and then injecting an electrolyte into the battery case.
However, in the case where a separator as proposed in the above cited patent references is used, it is certain that the problem of reduction of productivity of battery assembly due to the rise of the pressure required to incorporate the electrode group in battery as seen with the aforesaid conventional separator mainly composed of glass fibers alone and storage battery can be solved, but the incorporation of a minute hollow body in the gap between the glass fibers which are a main component of the separator causes the reduction of the porosity of the separator and hence the deterioration of the electrolyte retention, i.e., wicking ability or wicking volume of the separator.
Therefore, an object of the present invention is to provide a separator for storage battery mainly composed of microfibrous glass capable of enhancing productivity of battery assembly without deteriorating electrolyte retention, a storage battery comprising the aforesaid separator and a method of producing the same in order to eliminate the disadvantages of the above cited patent references.